Ditopic bis-terdentate cyclometallating ligands and their highly luminescent dinuclear iridium(iii) complexes

Article abstract of DOI:10.1039/c4cc01808g

A new family of bridged cyclometallating ligands is reported, which incorporate two terdentate N^∧C^∧N-coordinating binding sites linked via pyrazine, pyrimidine or pyridazine units. Dinuclear Ir(iii) complexes of one ligand have been prepared and crystallographically characterised; they display intense red phosphorescence. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01808g

Volume 50 Number 52 4 July 2014 Pages 6807–6936
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J. A. Gareth Williams, Valery N. Kozhevnikov et al.
Ditopic bis-terdentate cyclometallating ligands and their highly
luminescent dinuclear iridium(III) complexes

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Ditopic bis-terdentate cyclometallating ligands
and their highly luminescent dinuclear iridium(III)
complexes†
Cite this: Chem. Commun., 2014,
50, 6831
Received 10th March 2014,
Accepted 31st March 2014
Pierre-Henri Lanoe,a Chi Ming Tong,a Ross W. Harrington,b Michael R. Probert,b
¨
William Clegg,b J. A. Gareth Williams*c and Valery N. Kozhevnikov*a
DOI: 10.1039/c4cc01808g
www.rsc.org/chemcomm
A new family of bridged cyclometallating ligands is reported, which generate a plethora of structures with interesting geometries, rarely
incorporate two terdentate N4C4N-coordinating binding sites leads to systems that are strongly luminescent at room temperature,
linked via pyrazine, pyrimidine or pyridazine units. Dinuclear Ir(III) owing in part to the poor bite angle that leads to efficient non-
complexes of one ligand have been prepared and crystallographically radiative decay.7 In contrast, the isoelectronic N4C4N-binding
characterised; they display intense red phosphorescence.
ligand 1,3-bis(2-pyridyl)-benzene (dpyb)8 and its derivatives have
been used to prepare highly luminescent complexes of metals such
Multimetallic complexes formed by rigid ditopic ligands have as Ir(III), Rh(III) and Pt(II).9–11 The strong ligand field in combination
become well-known in contemporary coordination chemistry. For with high rigidity inhibits non-radiative decay, whilst efficient
example, polypyridine-type bridging ligands have underpinned the spin–orbit coupling pathways promote triplet phosphorescence.12
development of metallosupramolecular chemistry, leading to a wide Although such complexes have been incorporated as units into
variety of functional materials and unusual forms of chirality.1 multinuclear assemblies,13 the few examples reported to date fall
Amongst the many useful properties of polymetallic complexes are into the class of supramolecular system where the individual units
their increased extinction coefficients compared to mononuclear largely retain their original excited state properties, and energy
analogues and, frequently, their lower-energy absorption maxima.2 transfer occurs between them. In this contribution, we describe a
Such improved light-harvesting features are particularly attractive new family of rigid, bridged cyclometallating proligands that feature
in areas such as photoinduced electron- and energy-transfer for potential ditopic N4C4N–N4C4N coordination, and report on the
solar energy conversion and in photocatalysis.3 Meanwhile, for preparation and luminescent properties of mono- and dinuclear
luminescence, the presence of additional metal centres may facilitate Ir(III) complexes of one such ligand.
spin–orbit coupling (SOC) pathways and augment radiative rate
The synthetic strategy used to prepare the ligands is summarised
constants, increasing the efficiency of phosphorescence from triplet in Scheme 1, and is based on a combination of boronic acid
states, as required for use in organic light-emitting diodes.4 synthesis via ortho-lithiation and subsequent Pd-catalysed Suzuki
For example, efficient red emitters have been prepared using bis- cross-coupling reactions. The choice of central aryl synthon 1 was
cyclometallating ligands that bring together two N4C-coordinating made on the basis that: (i) the hexyl group would improve the
sites to bind simultaneously to Pt(II) and Ir(III) ions.5
solubility of the intermediates and final products, since one of the
It is widely recognised that terdentate ligands can offer structural drawbacks of rigid ligands is often poor solubility of products;
advantages over bidentate ligands, including the absence of chirality (ii) fluorine atoms are ortho-directing in the lithiation with n-BuLi,
in some cases and greater rigidity.6 However, the classic N4N4N- which will ensure the necessary regiochemistry for synthesis of
binding ligand 2,20:60,200-terpyridine, despite having been used to 1,3-diboronic acids; and (iii) the F atoms will block undesired,
competitive metallation of the C4 and C6 positions of the aryl ring
upon reaction with iridium(III) chloride, which is known to be the
a Department of Applied Sciences, Northumbria University, Newcastle upon Tyne,
predominant mode of binding for unsubstituted dpyb.10 The key
NE1 8ST, UK. E-mail: valery.kozhevnikov@northumbria.ac.uk
b School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
intermediate is the boronic acid 4, from which a variety of bridging
bis-terdentate N4C4N–N4C4N ligands can be prepared in one
step simply by reaction with an appropriate dihalogenated
heterocycle. For example, we prepared three such ligands L1H2,
L2H2 and L3H2 by the cross-coupling of 4 with 4,6-dichloro-
pyrimidine, 2,5-dibromopyrazine or 3,6-dichloropyridazine
respectively, under standard Suzuki conditions. Other linking
E-mail: bill.clegg@ncl.ac.uk
c Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE,
UK. E-mail: j.a.g.williams@durham.ac.uk
† Electronic supplementary information (ESI) available: Tables of crystallo-
graphic results, emission spectra of the Ir complexes at 77 K. CCDC 990003
and 990004. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4cc01808g
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metal ion, generating Ir2L1(ptpy)2Cl2. The coordination sphere is
completed by one monodentate chloride ligand remaining on
each metal. When the reaction was carried out with only one
equivalent of IrCl3ꢀ3H2O relative to L1H2, the corresponding
mononuclear complex IrL1H(ptpy)Cl was obtained, in which
L1H is singly metallated only.
Owing to the strong trans effect of metallated carbon atoms,
the ptpy ligand is introduced in such a way that its pyridyl ring,
not the aryl ring, is disposed trans to the central aryl ring of the
N4C4N unit.10,15 A single isomer of IrL1H(ptpy)Cl is therefore
isolated (as a racemic mixture of enantiomers). In the case of
the dinuclear Ir2L1(ptpy)2Cl2, three stereoisomers are formed:
one meso form with an internal mirror plane, and a racemic
pair of enantiomers in which the two chloride ligands are
disposed on opposite sides of the plane of L1 (Scheme 2). The
meso and rac isomers have different physical properties and
were easily separated by conventional column chromatography. The
isolated yields were in the region of 25% for each form. The
structures of the meso form and the rac pair were determined by
X-ray diffraction (Scheme 2).‡ The coordination of two Ir(III) cations
favours the adoption of a fully planar conformation by the bis-
terdentate ligand L1, and the Ir–Cl bonds are almost perpendicular
to the plane of L1. The L1 ligands in the two structures are essentially
identical apart from the hexyl substituents, and the main differences
are in the orientations of the ptpy ligands and the conformations of
the flexible hexyl chains, most of which display disorder.
Scheme 1 Synthesis of cyclometallating bis-terdentate ligands. (i) BuLi;
(ii) B(OiPr)3 then H3O+; (iii) 2-bromopyridine, Pd cat.; (iv) Pd cat.
heterocycles, such as naphthyridines or phenanthrolines, can
equally be introduced at this stage.
In the first instance, we have explored the complexation
chemistry of the pyrimidine-based proligand L1H2 with
iridium(III). 1,3-Di(2-pyridyl)-2,6-difluorobenzene (F2dpybH) is
known to react with IrCl3ꢀ3H2O in ethoxyethanol/water to give a
dichloro-bridged dimer [Ir(F2dpyb)Cl(m-Cl)]2.14 Applying the
same conditions to L1H2 with 2 equivalents of IrCl3ꢀ3H2O led to
a similarly bridged tetranuclear, dimeric complex [Ir2L1Cl2(m-Cl)2]2
in which, according to NMR, L1 is doubly metallated, bridging two
metal centres (Scheme 2). Treatment of this intermediate with
2-( p-tolyl)pyridine (ptpyH) in toluene in the presence of silver
triflate led to the cleavage of the chloro bridges and introduction
of a C4N-bound tolylpyridine into the coordination sphere of each
The absorption spectrum of the mononuclear complex
IrL1H(ptpy)Cl displays very intense bands in the UV region,
attributable to p–p* transitions within the ligand, together with
a series of moderately intense bands in the visible region
(Fig. 1, Table 1). By analogy with typical Ir(III) complexes with
cyclometallating aryl-heterocycle ligands,4d,16 the latter can be
attributed to singlet and triplet charge-transfer transitions,
involving filled orbitals localised predominantly on the metal
Scheme 2 Synthesis of Ir2L1(ptpy)2Cl2; schematic illustration of the relative disposition of the Cl ligands (centre), and corresponding structures obtained
by X-ray diffraction (right; for clarity, the structures are shown without H atoms, solvent molecules and minor disorder components).
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complex is in the orange-red region of the spectrum, peaking at
lmax = 595 nm, with a quantum yield of 0.59 and a luminescence
lifetime of 2.8 ms under deoxygenated conditions (Fig. 1,
Table 1). As in absorption, the introduction of the second metal
ion to generate meso- or rac-Ir2L1(ptpy)2Cl2 is accompanied by a
red shift (albeit a smaller one than in absorption), and the meso
and rac emission spectra are essentially identical to one another.
Remarkably, the quantum yield is enhanced for the dinuclear
complexes, despite the red shift. Typically, quantum yields tend
to drop off with decreasing excited state energy, as non-radiative
decay is facilitated. Some insight into the possible reasons for
the increase in F can be obtained by considering the rate
constants of radiative and non-radiative decay, kr and knr,
respectively. Assuming that the emissive excited state is formed
with unitary efficiency, the rate constants can be estimated from
Fig. 1 Absorption and photoluminescence spectra of IrL1H(ptpy)Cl (blue
lines) and the rac and meso forms of Ir2L1(ptpy)2Cl2 (green and red lines
respectively) in solution in CH2Cl2 at 298 K.
the lifetime and quantum yield as follows: kr = F/t and knr
=
tꢁ1 ꢁ kr. Such an analysis reveals that, although the non-radiative
and cyclometallating aryl rings, and vacant orbitals on the decay constants are indeed somewhat higher in the binuclear
heterocycle. It is notable, however, that these bands extend complexes, this is more than offset by a substantial 4-fold increase
to longer wavelength than the corresponding bands in, for in kr (Table 1).
example, Ir(F2dpyb)(ppy)Cl.14 This is readily rationalised on
The increase in kr upon introduction of the second metal ion
the basis that pyrimidine has a lower-energy p* orbital than is intriguing. Other things being equal, a decrease in the
pyridine and hence the energy of the charge-transfer transition radiative rate would be expected as the energy decreases,
will be lowered.
according to the Einstein coefficient which depends on n3.
The photophysical properties of the meso and rac forms of At least two effects may be at work here. Firstly, the fact that
Ir2L1(ptpy)2Cl2 are very similar to one another: their absorption T1 - S0 phosphorescence from such metal complexes is
spectra are almost identical, with differences in lmax values observed at room temperature is due to the high spin–orbit
being within the uncertainty of the measurement (Fig. 1). The coupling constant of the heavy metal ion (z = 3909 cmꢁ1 for Ir),
introduction of a second metal ion into the system – i.e. going from which breaks down the DS = 0 spin selection rule for radiative
IrL1H(ptpy)Cl to meso- or rac-Ir2L1(ptpy)2Cl2 – is accompanied by a decay. The presence of a second metal ion may be augmenting
large red shift of the bands in the visible region (e.g. the lowest- the spin–orbit coupling effect.5 Secondly, it may be noted that
energy band shifts by around 80 nm). Evidently, coordination of a the red-shift in emission on going from mono- to dinuclear
second Ir(III) cation to the other N atom of the pyrimidine ring complexes is smaller than the red-shift of the lowest-energy
will stabilise further the pyrimidine-based p* orbital, leading to absorption band, which indicates that the energy gap between
the observed red shift, irrespective of which isomer is formed. S1 and T1 is lower in the dinuclear systems. Using the lmax values
The behaviour in this respect is similar to that observed recently as a guide to the energies, we can estimate that DE(S1 ꢁ T1) is
for multinuclear complexes with bis-N4C-coordinating diphenyl- 3200 cmꢁ1 for IrL1H(ptpy)Cl but only around 1100 cmꢁ1 for
pyrimidine ligands,5 and is reminiscent of earlier studies on Ru(II) the dinuclear complexes. Spin–orbit coupling pathways are
complexes of dipyridylpyrimidine.2a
complicated and not fully understood, but it is well-
All three complexes are intensely photoluminescent in established that the T1 state must mix with higher-lying 1MLCT
solution at room temperature. The emission of the mononuclear states for phosphorescence to be promoted.17 The efficiency
Table 1 Photophysical properties of the iridium(III) complexesa
Emission at 77 Ke
em
l
/
kr/
knr/
max
Complex
Isomer lambasx/nm (e/Mꢁ1 cmꢁ1
)
nm
Fb
t/nsc
105 sꢁ1 d 105 sꢁ1 d
lmax/nm t/ns
IrL1H(ptpy)Cl
—
242 (68 100), 283 (57 100), 385 sh (9410),
402 (9700), 441 (14 000), 501 (5400)
250 (68 800), 287 (54 000), 339 (31 900), 398 (16 300), 622
448 (9930), 498 (18 900), 537 (10 800), 583 (7550)
250 (69 700), 286 (52 900), 340 (32 800), 399 (16 500), 625
447 (10 700), 497 (19 700), 537 (10 900), 582 (7420)
b
595
0.59 2800 [450] 2.1
0.65 760 [520] 8.6
0.65 730 [460] 8.9
1.5
4.6
4.8
562
13 000
3700
Ir2L1(ptpy)2Cl2 rac
617, 666
620, 672
Ir2L1(ptpy)2Cl2 meso
3500
a
In deoxygenated CH2Cl2 at 298 K except where stated otherwise. Luminescence quantum yield determined using Ru(bpy)3Cl2 in water as the
standard; estimated uncertainty in absolute values is ꢂ20% or better, while the error on relative values amongst the three complexes is o5%.
c
d
Values in parentheses refer to air-equilibrated solutions; estimated uncertainty in lifetime values is ꢂ10% or better. kr and knr estimated as
e
described in the text, assuming that the emissive state is formed with unitary efficiency upon light absorption. In diethyl ether/isopentane/
ethanol (2 : 2 : 1 v/v).
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4 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151; (b) Highly
Efficient OLEDs with Phosphorescent Materials, ed. H. Yersin,
Wiley-VCH, Weinheim, Germany, 2007; (c) Y. Chi and P. T. Chou,
Chem. Soc. Rev., 2010, 39, 638; (d) L. F. Gildea and J. A. G. Williams,
Iridium and platinum complexes for OLEDs, in Organic Light-Emitting
Diodes: Materials, Devices and Applications, ed. A. Buckley, Woodhead,
Cambridge, 2013.
with which this occurs is inversely proportional to the energy
gap between the T1 state and the higher singlet state. The
observation of a lower DE(S1 ꢁ T1) in the dinuclear complexes
may thus be indicative of more efficient SOC pathways.
In summary, we have presented a new family of cyclometallating
bis(N4C4N)-coordinating ligands and their application in the
synthesis of dinuclear Ir(III) complexes. The synthetic methodology,
centred around cross-couplings of synthon 4, allows facile variation
of the bridging unit within the ligands, and provides access to a
variety of possible structures. In comparison to classical coordina-
tion complexes of polypyridines, cyclometallating ligands offer both
structural and photophysical advantages. The strong trans effect of
cyclometallating carbon atoms dictates the orientation of the N4C
ligand in the coordination sphere, such that only those isomers in
which the heterocycle of the N4C ligand is trans to the carbon of
the N4C4N unit are formed. The di-iridium complexes are
amongst the very best phosphorescent red emitters known to
date.18 Phosphorescent emission at 625 nm with 65% quantum
yields and emission lifetimes o1 ms render these materials strong
candidates for consideration as OLED dopants. Moreover, the
strong absorption in the orange-to-red region is a highly desirable
attribute for sensitisers in both dye-sensitised solar cells and in
photocatalysed water splitting, where iridium(III) complexes are
attracting more and more attention.19 Thus, the structural and
emissive properties of this new class of complexes are of interest for
exploitation in metallo-supramolecular chemistry and the design of
new functional materials.
5 V. N. Kozhevnikov, M. C. Durrant and J. A. G. Williams, Inorg. Chem.,
¨
2011, 50, 6304; S. Culham, P.-H. Lanoe, V. L. Whittle, M. C. Durrant,
J. A. G. Williams and V. N. Kozhevnikov, Inorg. Chem., 2013,
52, 10992.
6 J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C. Coudret,
V. Balzani, F. Barigelletti, L. De Cola and L. Flamigni, Chem. Rev.,
1994, 94, 993.
¨
7 L. Hammarstrom, F. Barigelletti, L. Flamigni, M. T. Indelli,
N. Armaroli, G. Calogero, M. Guardigli, A. Sour, J. P. Collin and
J. P. Sauvage, J. Phys. Chem. A, 1997, 101, 9061; M. Maestri,
N. Armaroli, V. Balzani, E. C. Constable and A. M. W. Cargill
Thompson, Inorg. Chem., 1995, 34, 2759.
8 M. Beley, J. P. Collin, R. Louis, B. Metz and J. P. Sauvage, J. Am.
Chem. Soc., 1991, 113, 8521; M. Beley, J. P. Collin and J. P. Sauvage,
Inorg. Chem., 1993, 32, 4549.
9 J. A. G. Williams, Chem. Soc. Rev., 2009, 38, 1783.
10 A. J. Wilkinson, A. E. Goeta, C. E. Foster and J. A. G. Williams, Inorg.
Chem., 2004, 43, 6513; A. J. Wilkinson, H. Puschmann,
J. A. K. Howard, C. E. Foster and J. A. G. Williams, Inorg. Chem.,
2006, 45, 8685; L. F. Gildea, A. S. Batsanov and J. A. G. Williams,
Dalton Trans., 2013, 42, 10388.
11 S. J. Farley, D. L. Rochester, A. L. Thompson, J. A. K. Howard and
J. A. G. Williams, Inorg. Chem., 2005, 44, 9690; V. N. Kozhevnikov,
B. Donnio and D. W. Bruce, Angew. Chem., Int. Ed., 2008, 47, 6286;
S. Develay, O. Blackburn, A. L. Thompson and J. A. G. Williams,
Inorg. Chem., 2008, 47, 11129.
12 A. Rausch, L. Murphy, J. A. G. Williams and H. Yersin, Inorg. Chem.,
2012, 51, 312; A. Rausch, L. Murphy, J. A. G. Williams and H. Yersin,
Inorg. Chem., 2009, 48, 11407.
13 V. L. Whittle and J. A. G. Williams, Inorg. Chem., 2008, 47, 6596.
14 P. Brulatti, R. J. Gildea, J. A. K. Howard, V. Fattori, M. Cocchi and
J. A. G. Williams, Inorg. Chem., 2012, 51, 3813.
15 J. Kuwabara, T. Namekawa, M. Haga and T. Kanbara, Dalton Trans.,
2012, 41.
16 A. B. Tamayo, B. D. Alleyne, P. I. Djuorovich, S. Lamansky, I. Tsyba,
N. N. Ho, R. Bau and M. E. Thompson, J. Am. Chem. Soc., 2003,
125, 7377.
17 A. F. Rausch, H. H. H. Homeier, P. I. Djurovich, M. E. Thompson
and H. Yersin, Proc. SPIE-Int. Soc. Opt. Eng., 2007, 6655, F6550;
H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer,
Coord. Chem. Rev., 2011, 255, 2622; P. T. Chou, Y. Chi, M. W. Chung
and C. C. Lin, Coord. Chem. Rev., 2011, 255, 2653.
18 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am.
Chem. Soc., 2001, 123, 4304; A. Tsuboyama, H. Iwawaki, M. Furugori,
T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura,
T. Takiguchi, S. Okada, M. Hoshini and K. Ueno, J. Am. Chem.
Soc., 2003, 125, 12971; Y. J. Su, H. L. Huang, C. L. Li, C. H. Chien,
Y. T. Tao, P. T. Chou, S. Datta and R. S. Liu, Adv. Mater., 2003,
15, 224; F. M. Hwang, H. Y. Chen, P. S. Chen, C. S. Liu, Y. Chi,
C. F. Shu, F. I. Wu, P. T. Chou, S. M. Peng and G. H. Lee, Inorg.
Chem., 2005, 44, 1344; H. Bronstein, C. E. Finlayson, K. R. Kirov,
R. H. Friend and C. K. Williams, Organometallics, 2008, 27, 2980.
19 For a very recent example featuring structurally related mononuclear
Ir complexes, see: D. N. Chirdon, W. J. Transue, H. N. Kagalwala,
A. Kaur, A. B. Maurer, T. Pintauer and S. Bernhard, Inorg. Chem.,
2014, 53, 1487.
We thank EPSRC (grant ref. EP/I014942/1) for support of this
work, and Diamond Light Source for access to synchrotron
single-crystal diffraction beamline I19. Mass spectra were
acquired at the EPSRC UK National Mass Spectrometry Facility
at Swansea University.
Notes and references
‡ Data were collected at beamline I19 of Diamond Light Source
(l = 0.6889 Å). Structure solution and refinement20 included modelling
of disorder for 3 of the 4 hexyl chains, inclusion of ordered solvent
molecules in both structures with the aid of restraints, and SQUEEZE
treatment21 of additional disordered solvent in the meso structure.
1 G. S. Hanan, C. R. Arana, J.-M. Lehn and D. Fenske, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 1122; M. W. Cooke, D. Chartrand and
G. S. Hanan, Coord. Chem. Rev., 2008, 252, 903.
2 For example: (a) S. Serroni, A. Juris, S. Campagna, M. Venturi, G. Denti
and V. Balzani, J. Am. Chem. Soc., 1994, 116, 9086; (b) P. Ceroni, A. Credi,
V. Balzani, S. Campagna, G. S. Hanan, C. R. Arana and J.-M. Lehn, Eur.
J. Inorg. Chem., 1999, 1409; (c) A. Credi, V. Balzani, S. Campagna,
G. S. Hanan, C. R. Arana and J.-M. Lehn, Chem. Phys. Lett., 1995,
243, 102; (d) G. S. Hanan, C. R. Arana, J.-M. Lehn, G. Baum and
D. Fenske, Chem. – Eur. J., 1996, 2, 1292–1302.
3 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev.,
2010, 110, 6595; N. Kaveevivitchai, R. Chitta, R. Zong, M. El Ojaimi and
R. P. Thummel, J. Am. Chem. Soc., 2012, 134, 10721; Z. Deng,
H.-W. Tseng, R. Zong, D. Wang and R. Thummel, Inorg. Chem., 2008, 20 SHELXTL, Bruker AXS Inc., Madison, WI, USA, 2013; G. M. Sheldrick,
¨
47, 1835; V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and
M. Venturi, Acc. Chem. Res., 1998, 31, 26.
SHELXL-2013, University of Gottingen, 2013.
21 A. L. Spek, Acta Crystallogr., Sect. D, 2009, 65, 148.
6834 | Chem. Commun., 2014, 50, 6831--6834
This journal is ©The Royal Society of Chemistry 2014